Mineral oil lubricants are derived from various crude oil stocks by a variety of refining processes. Generally, these refining processes are directed towards obtaining a lubricant base stock of suitable boiling point, viscosity, viscosity index (VI) and other characteristics. Generally, the base stock will be produced from the crude oil by distillation of the crude in atmospheric and vacuum distillation towers, followed by the separation of undesirable aromatic components and finally, by dewaxing and various finishing steps. Because aromatic components lead to high viscosity and extremely poor viscosity indices, the use of asphaltic type crudes is not preferred as the yield of acceptable lube stocks will be extremely low after the large quantities of aromatic components contained in such crudes have been separated out; paraffinic and naphthenic crude stocks will therefore be preferred but aromatic separation procedures will still be necessary in order to remove undesirable aromatic components. In the case of the lubricant distillate fractions, generally referred to as the neutrals, e.g. heavy neutral, light neutral, etc., the aromatics will be extracted by solvent extraction using a solvent such as furfural, N-methyl-2-pyrrolidone phenol or another material which is selective for the extraction of the aromatic components. If the lube stock is a residual lube stock, the asphaltenes will first be removed in a propane deasphalting step followed by solvent extraction of residual aromatics to produce a lube generally referred to as bright stock. In either case, however, a dewaxing step is normally necessary in order for the lubricant to have a satisfactorily low pour point and cloud point, so that it will not solidify or precipitate the less soluble paraffinic components under the influence of low temperatures.
A number of dewaxing processes are known in the petroleum refining industry and of these, solvent dewaxing with solvents such as methylethylketone (MEK), a mixture of MEK and toluene or liquid propane, has been the one which has achieved the widest use in the industry. Recently, however, proposals have been made for using catalytic dewaxing processes for the production of lubricating oil stocks and these processes possess a number of advantages over the conventional solvent dewaxing procedures. The catalytic dewaxing processes which have been proposed are generally similar to those which have been proposed for dewaxing the middle distillate fractions such as heating oils, jet fuels and kerosenes, of which a number have been disclosed in the literature, for example, in Oil and Gas Journal, Jan. 6, 1975, pp. 69-73 and U.S. Pat. Nos. RE 28,398, 3,956,102 and 4,100,056. Generally, these processes operate by selectively cracking the normal and slightly branched paraffins to produce lower molecular weight products which may then be removed by distillation from the higher boiling lube stock. The catalysts which have been proposed for this purpose have usually been zeolites which have a pore size which admits the straight chain, waxy n-paraffins either alone or with only slightly branched chain paraffins but which exclude more highly branched materials and cycloaliphatics. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-38 and the synthetic ferrierites have been proposed for this purpose in dewaxing processes, as described in U.S. Pat. Nos. 3,700,585 (Re 28398); 3,894,938; 3,933,974; 4,176,050; 4,181,598; 4,222,855; 4,259,170; 4,229,282; 4,251,499; 4,343,692, and 4,247,388. A dewaxing process employing synthetic offretite is described in U.S. Pat. No. 4,259,174. Processes of this type have become commercially available as shown by the 1986 Refining Process Handboook, Hydrocarbon Processing, September 1986, which refers to the availability of the Mobil Lube Dewaxing Process (MLDW). Reference is made to these disclosures for a description of various catalytic dewaxing processes.
With the catalytic dewaxing processes of the type described above where the dewaxing is effected by a shape selective cracking of the waxy paraffinic components in the feed, extended catalyst cycle life is generally achieved without difficulty. However, in certain instances, problems may be encountered. For example, if the feed contains certain contaminents which affect the catalyst activity adversely, it may be desirable to subject the feed to an initial contaminent removal step by sorption over a zeolite in order to remove these contaminents. A process of this kind is described in U.S. Pat. Nos. 4357232 and a similar process for treating waxy fuel oils is described in U.S. Pat. No. 4358363. Typical aging curves for an intermediate pore size dewaxing catalyst are shown in U.S. Pat. No. 3956102 and U.S. Pat. No. 3894938 discloses that the cycle life of an intermediate pore size dewaxing catalyst may be longer with a virgin feed stream than it is with the same feed stream after it has been hydrotreated. These and other problems are encountered most frequently with lube boiling feeds and this has tended to retard the spread of catalytic lube dewaxing processes. While there are probably hundreds of solvent dewaxing units operating, only seven catalytic lube dewaxers are believed so far to be operating (end 1986).
As stated above, catalytic dewaxing processes of this type operate by selective cracking of the waxy components in the feed. This implies that when the feed contains a relative high quantity of waxy components, the catalyst must be operated under conditions of relatively greater severity in order to achieve the target pour point. The increasing severity of operation, however, may lead to unacceptably short cycle times between successive catalyst reactivations because the high level of paraffin cracking which takes place under these conditions tends to deposit coke on the catalyst more rapidly than usual so that the catalyst quickly becomes deactivated and the operating temperature required to achieve the target pour point may increase excessively. It is, of course, desirable to avoid excessively high temperatures during any cycle since at these higher temperatures non-selective thermal and catalytic cracking becomes more favored. In certain cases, cycle life may become extremely short and may even become as short as a matter of a few hours which is quite unacceptable for commercial operation.
It would be possible to maintain catalyst activity by carrying out reactivation or regeneration at frequent intervals but although this may be acceptable for laboratory scale studies, it is quite unsatisfactory for commercial operation because it requires larger amounts of the relatively expensive dewaxing catalyst to be employed so that reactivation or regeneration can be carried out while dewaxing is proceeding with another load of catalyst.
In Application Ser. No. 087,198 (Mobil Case 4316) there is described a dewaxing process which achieves significantly longer catalyst life between successive restorative treatments by carrying out the dewaxing in two steps with the first step at substantially constant temperature during each dewaxing cycle. Although the dewaxing activity of the catalyst progressively decreases during the cycle, no attempt is made to obtain a fixed pour point in the first step. The target pour point for the product is obtained in the second step by progressively increasing the temperature as the catalyst ages. Thus, the process is carried out under differential severity condition as the catalysts age during each dewaxing cycle.
Reference is made to Ser. No. 087,198 (Mobil Case 4316) for a full description of the dewaxing process.